Use browser top chrome material design by default on Chrome OS

ui/base/material_design/material_design_controller.h

Index: ui/base/material_design/material_design_controller.h
diff --git a/ui/base/material_design/material_design_controller.h b/ui/base/material_design/material_design_controller.h
index 8f6af4177b57b98ca528ba79ab55972b6a65c45d..9ef61c0c4e546250afeb927a3f1c9a4c39c5673c 100644
--- a/ui/base/material_design/material_design_controller.h
+++ b/ui/base/material_design/material_design_controller.h
@@ -34,6 +34,9 @@ class UI_BASE_EXPORT MaterialDesignController {
   // Returns true if the current mode is a material design variant.
   static bool IsModeMaterial();
 
+  // Returns the per-platform default material design variant.
+  static Mode DefaultMode();
+
  private:
   friend class test::MaterialDesignControllerTestAPI;